Confined Defect Profiling within Resistive Random Memory Access Cells

ABSTRACT

Provided are resistive random access memory (ReRAM) cells and methods of fabricating thereof. A stack including a defect source layer, a defect blocking layer, and a defect acceptor layer disposed between the defect source layer and the defect blocking layer may be subjected to annealing. During the annealing, defects are transferred in a controllable manner from the defect source layer to the defect acceptor layer. At the same time, the defects are not transferred into the defect blocking layer thereby creating a lowest concentration zone within the defect acceptor layer. This zone is responsible for resistive switching. The precise control over the size of the zone and the defect concentration within the zone allows substantially improvement of resistive switching characteristics of the ReRAM cell. In some embodiments, the defect source layer includes aluminum oxynitride, the defect blocking layer includes titanium nitride, and the defect acceptor layer includes aluminum oxide.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to U.S. Provisional Application Ser.No, 61/782,609 filed on Mar. 14, 2013, which is herein incorporated byreference for all purposes.

BACKGROUND

Nonvolatile memory is computer memory capable of retaining the storedinformation even when unpowered. Non-volatile memory may be used forsecondary storage or long-term persistent storage, for example, inaddition to volatile memory. Nonvolatile memory can be permanentlyintegrated into computer systems (e.g., solid state hard drives) or itcan take the form of removable and easily transportable memory cards(e.g., USB flash drives). Nonvolatile memory is becoming more popularbecause of its small size and high density, low power consumption, fastread and write rates, data retention, and other characteristics.

Flash memory is a common type of nonvolatile memory because of its highdensity and low fabrication costs. Flash memory is a transistor-basedmemory device that uses multiple gates per transistor and quantumtunneling for storing the information on its memory device. Flash memoryuses a block-access architecture that can result in long access, erase,and write times. Flash memory also suffers from low endurance, highpower consumption, and scaling limitations.

The constantly increasing speed of electronic devices and storage demanddrive new requirements for nonvolatile memory. For example, nonvolatilememory is expected to replace hard drives in many new computer systems.However, transistor-based flash memory is often inadequate to meet therequirements for nonvolatile memory. New types of memory, such asresistive random access memory (ReRAM), are being developed to meetthese demands and requirements.

SUMMARY

Provided are resistive random access memory (ReRAM) cells and methods offabricating thereof. A stack including a defect source layer, a defectblocking layer, and a defect acceptor layer disposed between the defectsource layer and the defect blocking layer may be subjected toannealing. During the annealing, defects are transferred in acontrollable manner from the defect source layer to the defect acceptorlayer. At the same time, the defects are not transferred into the defectblocking layer thereby creating a lowest concentration zone within thedefect acceptor layer. This zone is responsible for resistive switching.The precise control over the size of the zone and the defectconcentration within the zone allows improvement of resistive switchingcharacteristics of the ReRAM cell. In some embodiments, the defectsource layer includes aluminum oxynitride, the defect blocking layerincludes titanium nitride, and the defect acceptor layer includesaluminum oxide.

In some embodiments, a method of fabricating a ReRAM cell involvesproviding a stack that include a defect source layer, a defect blockinglayer, and a defect acceptor layer. In this stack, the defect acceptorlayer is disposed between the defect source layer and the defectblocking layer. The method continues with annealing, which transfersdefects from the defect source layer to the defect acceptor layer. Thistransfer creates a resistive switching zone within the defect acceptorlayer. The resistive switching zone has a predetermined concentration ofdefects. No defects are transferred from the defect acceptor layer tothe defect blocking layer during this last operation, i.e., transferringdefects from the defect source layer to the defect acceptor layer. Insome embodiments, this operation involves annealing the stack at atemperature of between 500° C. and 800° C. for between 30 seconds and 30minutes. For example, a standard activation anneal of 750° C. for 1minute may be used.

Some examples of defects include oxygen, nitrogen, carbon, and silicon.In some embodiments, defects are provided within the defect blockinglayer. For example, the defect blocking layer may be formed fromtitanium nitride and nitrogen may be used as defects. Some of thedefects in the defect blocking layer may be transferred to the defectsource layer. In some embodiments, the defect blocking layer includes ametal and substantially no metal is transferred from the defect blockinglayer to the defect source layer. In other words, the metal in thedefect blocking layer does not become a source of defects within theresistive switching zone. This approach is contrary to many conventionalapproaches, in which metal layers are used as a source of defects, e.g.,as a source of oxygen vacancies.

In some embodiments, the defect blocking layer may be one of siliconoxide, aluminum oxide, hafnium oxide, zirconium oxide, titanium nitride,tantalum nitride, or aluminum nitride. The defect blocking layer may beoperable as an electrode. In some embodiments, the defect acceptor layerand the defect source layer include the same metal. The defect sourcelayer may include one of AlO_(X)N_(Y), HfO_(X)N_(Y), ZrO_(X)N_(Y),SiO_(X)N_(Y), TiO_(X), or TaO_(X). For example, the defect source layerincludes AlO_(X)N_(Y), while the defect acceptor layer includes Al₂O₃prior to the annealing.

In some embodiments, the defect source layer includes between about 10atomic percent and 30 atomic percent of nitrogen on average prior to theannealing. This nitrogen may be distributed non-uniformly throughout athickness of the defect source layer prior to the annealing. Forexample, more nitrogen may be present within the defect source layer atits interface with the defect acceptor layer than elsewhere in thedefect source layer prior to the annealing.

The defect source layer and the defect acceptor layer are used to form aresistive switching zone by annealing, which transfers defects from thedefect source layer and into the defect acceptor layer. In someembodiments, the resistive switching zone includes between about 1atomic percent and 15 atomic percent of nitrogen on average after theannealing or, more specifically, less than 10 atomic percent of nitrogenon average. The resistive switching zone may have the lowest nitrogenconcentration within the defect acceptor layer and the defect sourcelayer. The resistive switching zone may include the lowest nitrogenconcentration within the defect acceptor layer and the defect blockinglayer.

In some embodiments, a method of fabricating a resistive random accessmemory cell involves providing a first layer including aluminum oxide.The first layer may have a thickness of between about 20 Angstroms andabout 100 Angstroms. The method may proceed with introducing nitrogeninto the first layer. This operation may involve plasma nitridationusing ammonia. The first layer may include between about 15 atomicpercent and 25 atomic percent on average after introducing nitrogen. Themethod may proceed with depositing a second layer including aluminumoxide over the first layer. The second layer may have a thickness ofbetween about 20 Angstroms to about 100 Angstroms. The method mayproceed with depositing a third layer including titanium nitride overthe second layer. The method may proceed with annealing the first layer,the second layer, and the third layer at between about 700° C. and 800°C. for between about 0.5 minutes and 2 minutes. Annealing results intransferring nitrogen from the first layer and into the second layerwhile no nitrogen is transferred from the second layer and into thethird layer. After annealing, the second layer has a lowest nitrogenconcentration zone between the first layer and the second layer. Thelowest nitrogen concentration zone may have a nitrogen concentration ofless than 10 atomic percent.

Provided also a ReRAM cell including a first layer having aluminum,oxygen, and nitrogen. The first layer may have a thickness of betweenabout 20 Angstroms to about 100 Angstroms. The ReRAM call also includesa second layer including aluminum, oxygen, and nitrogen. The secondlayer disposed over the first layer. The second layer may have athickness of between about 20 Angstroms to about 100 Angstroms. TheReRAM cell also includes a third layer having titanium nitride. Thethird layer is disposed over the second layer. The second layer has alowest nitrogen concentration zone within the first layer and the secondlayer. The lowest nitrogen concentration zone has a nitrogenconcentration of less than 10 atomic percent.

BRIEF DESCRIPTION OF THE DRAWINGS

To facilitate understanding, the same reference numerals have been used,where possible, to designate common components presented in the figures.The drawings are not to scale and the relative dimensions of variouselements in the drawings are depicted schematically and not necessarilyto scale. Various embodiments can readily be understood by consideringthe following detailed description in conjunction with the accompanyingdrawings, in which:

FIG. 1A illustrate schematic representations of a ReRAM cell prior toinitial forming operation, in accordance with some embodiments.

FIGS. 1B and 1C illustrate schematic representations of the ReRAM cellin its high resistive state (HRS) and low resistive state (LRS), inaccordance with some embodiments.

FIG. 2A illustrates a plot of a current passing through a unipolar ReRAMcell as a function of a voltage applied to the ReRAM cell, in accordancewith some embodiments.

FIG. 2B illustrates a plot of a current passing through a bipolar ReRAMcell as a function of a voltage applied to the ReRAM cell, in accordancewith some embodiments.

FIG. 3 illustrates a schematic representation of a ReRAM cell includinga defect source layer, a defect blocking layer, and a defect acceptorlayer disposed between the defect source layer and the defect blockinglayer, in accordance with some embodiments.

FIG. 4 illustrates a process flowchart corresponding to a method offorming a ReRAM cell, in accordance with some embodiments.

FIGS. 5A and 5B illustrate a schematic representation of a ReRAM cellbefore and after annealing, which transfers defects from the defectsource layer to the defect acceptor layer, in accordance with someembodiments.

FIGS. 5C and 5D are illustrative defect concentration profiles as afunction of the location within the ReRAM cell before and afterannealing, which transfers defects from the defect source layer to thedefect acceptor layer, in accordance with some embodiments.

FIG. 5E illustrates a schematic representation of a ReRAM cell that doesnot include a defect blocking layer, in accordance with someembodiments.

FIG. 5F is an illustrative defect concentration profile as a function ofthe location within a ReRAM cell that does not include a defect blockinglayer, in accordance with some embodiments.

FIGS. 6A and 6B illustrate schematic views of memory arrays includingmultiple ReRAM cells, in accordance with some embodiments.

FIG. 7 illustrates plots of a nitrogen concentration and an oxygenconcentration as a function of the position within a test cell after itsannealing.

DETAILED DESCRIPTION

A detailed description of various embodiments is provided below alongwith accompanying figures. The detailed description is provided inconnection with such embodiments, but is not limited to any particularexample. The scope is limited only by the claims and numerousalternatives, modifications, and equivalents are encompassed. Numerousspecific details are set forth in the following description in order toprovide a thorough understanding. These details are provided for thepurpose of example and the described techniques may be practicedaccording to the claims without some or all of these specific details.For the purpose of clarity, technical material that is known in thetechnical fields related to the embodiments has not been described indetail to avoid unnecessarily obscuring the description.

Introduction

A memory cell exhibiting resistive switching characteristics, which isoften referred to as a ReRAM cell, generally includes multiple layersformed into a stack. The overall structure of this stack is sometimesdescribed as a Metal-Insulator-Metal (MIM). Specifically, the stackincludes two conductive layers operating as electrodes. These layers areidentified as “M” and may include metals and/or other conductivematerials, such as titanium nitride or doped polysilicon. The stack alsoincludes one or more insulator layers provided in between theelectrodes. These layers may be collectively identified as “I” in theabove naming convention. The insulator layer or a portion withinmultiple layers exhibits resistive switching properties characterized bydifferent resistive states of the material forming this layer or theportion. As such, this insulator layer may be referred to as a resistiveswitching layer. These resistive states may be used to represent one ormore bits of information. Each of these three main layers may includeany number of sub-layers. Furthermore, a ReRAM may include additionallayers as further described below.

The ReRAM cell changes its resistive state by applying one or morevoltage pulses (e.g., set voltage pulses to go from a higher resistanceto a lower resistance or reset voltage pulses to go from a lowerresistance to a high resistance) to the cell as further explained below.The applied voltage causes some localized heating within the switchingzone within the cell. Without being restricted to any particular theory,it is believed that a combination of the electrical field and localizedheating (both created by the applied voltage) causes formation andbreakage of various conductive paths within the switching zone. It isbelieved that various defects change their location forming theconductive paths (when the set voltage is applied) and breaking theconductive paths (when the reset voltage is applied) and causing changesin the resistance. For example, an aluminum oxide layer may include somenitrogen that can form conductive paths within the layer.

For purposes of this disclosure, defects are defined as any materials orvacancies that can change their orientations within the switching zonebased on applied voltage. This change in orientation causes formation ofthe conductive paths or breaking of the conductive paths, which resultsin changes of the resistance. Not all materials or vacancies present inthe switching zone may function as defects. Some materials or vacanciesmay not be sufficiently mobile, while mobility of other materials orvacancies may not impact the resistance. The defects should besufficiently mobile materials or vacancies. For example, a lowconcentration (e.g., less than 10 atomic percent or even less than 5atomic percent) of nitrogen in an aluminum oxide layer may be useddefects.

Typically, only one type of defect is used within one layer. Havingmultiple types of defects operating simultaneously within the switchingzone may interfere with resistive switching characteristics and may bedifficult, if possible at all to control. The defects may be introducedby doping a base material, which may be an oxide and/or a nitride of oneor more transition metals. Dopant atoms replace base material atoms inthe original base material structure (e.g., its lattice) causingstresses within this structure. These stresses make it is easier to formconductive paths through the doped structure than, for example, to formsimilar paths through an undoped structure.

Defects should be introduced into a switching zone in a controlledmanner. The amount of defects in this zone should be sufficient to formsome conductive paths during a set operation, i.e., going from a lowresistance to a high resistance. At the same time, the level of defectsshould not be excessive in order to be able break these paths during areset operation, i.e., going from a high resistance to a low resistance.In other words, if the switching zone does not have enough defects, thenthe conductive paths may be hard to form if these paths can be formed atall. Excessive set voltages may be needed to achieve an adequate changein resistance. On the other hand, if the switching zone has too manydefects, then the conductive paths may be difficult to break as thereare many conductive paths capable of conductive electrical current andlarger overall power may be required.

As such, tight control of the amount of defects available within theswitching zone may help to reduce switching voltages (i.e., set voltagesand reset voltages) and reduce the overall power required to operate theReRAM cell. Another consideration for lowering the switching voltagesand operating power is the thickness of the layers of the ReRAM cell andin particular the thickness of the switching zone. Without beingrestricted to any particular theory, it is believed that thinner layersrequire lower switching voltages. In some embodiments, the layers usedto form and contain the zone may be less than 100 Angstroms thick, forexample, 50 Angstroms thick. Overall, ReRAM cell performance heavilydepends on the amount of defects and their distribution profiles withinthe cell.

Precisely controlling distribution of defects in such thin layer ischallenging. Furthermore, the switching zone should be well defined anddifferentiated from other portions within the ReRAM cell so that theseother portions do not exhibit resistive switching characteristics. Theswitching zone may be defined by maintaining higher concentrations ofdefects in the portions of the cell that may be susceptible to resistiveswitching (i.e., have some defect mobility). While these other portionsmay exhibit some defect mobility, the change in resistance caused bythis mobility is negligible because of relatively high concentration ofdefects. The switching zone and these other portions of the cell may becollectively referred to as a resistive switching layer and should bedistinguished from other components, such as electrodes, currentlimiting layer, current steering elements, which generally do notexhibit any defect mobility. It should be noted however that some ofthese other components may still include defects, but these defects arerelative immobile in these components.

The challenge in controlling distribution of defects within ReRAM cellsis due to very thin layers used for these cells. Furthermore, ReRAMcells are often subject to high temperatures, e.g., used for annealingand activating other components, such as diodes. Materials in thin films(e.g., less 50 Angstroms) readily intermix when exposed to hightemperatures that may result in uniform distribution of defect anddifficulties in defining a switching zone. For example, when metallayers are used to introduce oxygen vacancies into resistive switchinglayers, with which the metal layers interface, the oxygen vacancies tendto have gradual distribution from one side of the resistive switchinglayer to another side. This phenomenon is described in more details withreference to FIGS. 5E and 5F below. It may be desirable to have aswitching zone that has a lower concentration of defects than twoadjacent portions. In other words, the switching zone may be defined bya drop of the defect concentration or so called a low defectconcentration well. The gradual distribution of defects does not providea similar switching effect and result in generally high switchingvoltages.

Furthermore, metals can easily diffuse through various layers of theReRAM cells. Very few materials are effective blockers of metals whensubjected to annealing and resistive switching conditions. Withoutboundary for defect movement, there is very little control that can beexerted on defect distribution.

Provided are novel ReRAM cells and methods of controlling defectdistribution within the ReRAM cells. The controlled distribution isachieved by selecting specific materials for different components of theReRAM cells, such as a defect source layer, a defect blocking layer, anda defect acceptor layer. The components are spatially arranged in aparticular manner that creates a well-defined switching zone having thelowest defect concentration after annealing the cells. Furthermore,specific types of defects are used in these cells, such as elements fromgroups IVA, VA, and VIA of the periodic table with specific examplesincluding carbon, nitrogen, oxygen, and silicon. A combination of thesefeatures allows achieving controlled mobility of defects between theReRAM cell components and within these components. In some embodiments,only one type of defect or, more specifically, only one element (e.g.,nitrogen) is allowed to move between and within the components. Otherelements may not move because of their uniform distribution (e.g., twolayer have substantially the same concentration of elements) or becausethey are locked within the layer (e.g., titanium in a titanium nitridelayer).

Each ReRAM cell includes a defect source layer, a defect blocking layer,and a defect acceptor layer that collectively form and are referred toas a stack. In this stack, the defect acceptor layer is disposed betweenthe defect source layer and the defect blocking layer. The defectblocking layer may also function as an electrode. The ReRAM cell mayalso include other layers, such as another electrode, current limiters,current steering elements, and the like.

When the stack is subjected to annealing or any other process thatinvolves raising the temperature of the stack, the defects flow from thedefect source layer and into the defect acceptor layer. This flow may bedriven by the concentration gradient and, therefore, may be referred toas diffusion. The amount of defects transferred from the defect sourcelayer and into the defect acceptor layer depends on the compositions,materials, and thicknesses of these two layers, temperature, time, andother parameters. However, the defect acceptor layer does not pass anydefects into the defect blocking layer. In fact, in some embodiments,the defect blocking layer may release some defects into the defectsacceptor layer. As such, a distinct zone in the defect acceptor layer isformed that has the lowest and controlled concentration of the defects.This distinct zone is believed to be responsible for resistive switchingand may be tuned to use lower switching voltages than, for example, whenconventional methods of introducing defects into a resistive switchinglayer are used. This zone is also referred to as a switching zone.

An example of a ReRAM cell will now be described to provide a betterunderstanding of ReRAM cell structure and methods of forming andoperating such structures. A layer of aluminum oxide (Al₂O₃) may beformed on a substrate including an electrode. The aluminum oxide layermay have a thickness of between about 50 Angstroms and 100 Angstroms.The first electrode could be any conductive material, such as dopedpolysilicon or, more specifically, n-doped polysilicon. Plasmanitridation or any other suitable technique may be used to introducenitrogen into aluminum oxide to generate defects in this layer. Whilethis modified layer now containing aluminum oxynitride can be used as aresistive switching layer, the distribution of nitrogen within the layeris gradual and will become even more gradual with subsequent annealing,which is not desirable. Furthermore, controlling the amount of nitrogenintroduced into this layer may be difficult.

The ReRAM fabrication process in the above examples then continues withforming a new layer of aluminum oxide (Al₂O₃) over the aluminumoxynitride layer and then forming a layer of titanium nitride over thenew aluminum oxide layer. In other words, the new aluminum oxide layeris disposed between the aluminum oxynitride layer and the titaniumnitride layer. In this stack, the aluminum oxynitride layer functions asa defect source layer, the new aluminum oxide layer functions as adefect acceptor layer, and the titanium nitride layer functions as adefect blocking layer. The stack including these three layers is thenannealed (e.g., in order to activate diodes provided on the samesubstrate), which causes nitrogen to transfer from the aluminumoxynitride layer to the new aluminum oxide layer. At the same time,nitrogen does not flow from the new aluminum oxide layer and into thetitanium nitride layer. In fact, some nitrogen may be transferred fromthe titanium nitride layer and into the new aluminum oxide layer. As aresult of this nitrogen transfer, the nitrogen concentration profile hasa minimum within the new aluminum oxide layer. This portion of the newaluminum oxide layer becomes responsible for resistive switching and maybe referred to as a switching zone. A series of experiments furtherdescribed below have shown that a reset voltage for a ReRAM cellprepared in accordance with an example presented above is about 5.8V,while a reset voltage for a ReRAM cell in which the aluminum oxynitridelayer is used as a resistive switching layer alone (i.e., without addinga new aluminum oxide layer) is about 7.6V.

Examples of Nonvolatile ReRAM Cells and their Switching Mechanisms

A brief description of ReRAM cells is provided for context and betterunderstanding of various features associated with using defect blockinglayers in the ReRAM cells. As stated above, a ReRAM cell includes adielectric material exhibiting resistive switching characteristics. Adielectric, which is normally insulating, can be made to conduct throughone or more conductive paths formed after application of a voltage. Theconductive path formation can arise from different mechanisms, includingdefects, metal migration, and other mechanisms further described below.Once one or more conductive paths (e.g., filaments) are formed in thedielectric component of a memory device, these conductive paths may bereset (or broken resulting in a high resistance) or set (or re-formedresulting in a lower resistance) by applying certain voltages. Withoutbeing restricted to any particular theory, it is believed that resistiveswitching corresponds to migration of defects within the resistiveswitching layer and, in some embodiments, across one interface formed bythe resistive switching voltage, when a switching voltage is applied tothe layer.

FIG. 1A illustrates a schematic representation of ReRAM cell 100including top electrode 102, bottom electrode 106, and resistanceswitching layer 104 disposed in between top electrode 102 and bottomelectrode 106. It should be noted that the “top” and “bottom” referencesfor electrodes 102 and 106 are used solely for differentiation and notto imply any particular spatial orientation of these electrodes. Oftenother references, such as “first formed” and “second formed” electrodesor simply “first” and “second”, are used identify the two electrodes.ReRAM cell 100 may also include other components, such as an embeddedresistor, diode, diffusion barrier layer, and other components. ReRAMcell 100 is sometimes referred to as a memory element or a memory unit.

Top electrode 102 and bottom electrode 106 may be used as conductivelines within a memory array or other types of devices that ReRAM cell isintegrated into. As such, electrode 102 and 106 are generally formedfrom conductive materials. As stated above, one of the electrodes may bereactive electrode and act as a source and as a reservoir of defects forthe resistive switching layer. That is, defects may travel through aninterface formed by this electrode with the resistive switching layer(i.e., the reactive interface).

Resistance switching layer 104 which may be initially formed from adielectric material and later can be made to conduct through one or moreconductive paths formed within the layer by applying first a formingvoltage and then a switching voltage. To provide this resistiveswitching functionality, resistance switching layer 104 includes aconcentration of electrically active defects 108, which may be at leastpartially provided into the layer during its fabrication. For example,some atoms may be absent from their native structures (i.e., creatingvacancies) and/or additional atoms may be inserted into the nativestructures (i.e., creating interstitial defects). Charge carriers may bealso introduced as dopants, stressing lattices, and other techniques.Regardless of the types all charge carriers are referred to as defects108.

FIG. 1A is a schematic representation of ReRAM cell 100 prior to initialformation of conductive paths, in accordance with some embodiments.Resistive switching layer 104 may include some defects 108. Additionaldefects 108 may be provided within top electrode 102 and may be latertransferred to resistive switching layer 104 during the formationoperation. In some embodiments, the resistive switching layer 104 hassubstantially no defects prior to forming operation and all defects areprovided from top electrode 102 during forming. Bottom electrode 106 mayor may not have any defects. It should be noted that regardless ofpresence or absence of defects in bottom electrode 106, substantially nodefects are exchanged between bottom electrode 106 and resistiveswitching layer 104 during forming and/or switching operations.

During the forming operation, ReRAM cell 100 changes its structure fromthe one shown in FIG. 1A to the one shown in FIG. 1B. This changecorresponds to defects 108 being arranged into one or more continuouspaths within resistive switching layer 104 as, for example,schematically illustrated in FIG. 1B. Without being restricted to anyparticular theory, it is believed that defects 108 can be reorientedwithin resistance switching layer 104 to form these conductive paths as,for example, schematically shown in FIG. 1B. Furthermore, some or alldefects 108 forming the conductive paths may enter resistive switchinglayer 104 from top electrode 102. For simplicity, all these phenomenaare collectively referred to as reorientation of defects within ReRAMcell 100. This reorientation of defects 108 occurs when a certainforming voltage is applied to electrodes 102 and 106. In someembodiments, the forming operation also conducted at elevatedtemperatures to enhanced mobility of the defects within ReRAM cell 100.In general, the forming operation is considered to be a part of thefabrication of ReRAM cell 100, while subsequent resistive switching isconsidered to be a part of operation of ReRAM cell.

Resistive switching involves breaking and reforming conductive pathsthrough resistive switching layer 104, i.e., switching between the stateschematically illustrated in FIG. 1B and the state schematicallyillustrated in FIG. 1C. The resistive switching is performed by applyingswitching voltages to electrodes 102 and 106. Depending on magnitude andpolarity of these voltages, conductive path 110 may be broken or formedback again. These voltages may be substantially lower than formingvoltages (i.e., voltages used in the forming operation) since much lessmobility of defects is needed during switching operations. For example,hafnium oxide based resistive layers may need about 7 Volts during theirforming but can be switched using voltages less than 4 Volts.

The state of resistive switching layer 104 illustrated in FIG. 1B isreferred to as a low resistance state (LRS), while the state illustratedin FIG. 1C is referred to as a high resistance state (HRS). Theresistance difference between the LRS and HRS is due to different numberand/or conductivity of conductive paths that exists in these states,i.e., resistive switching layer 104 has more conductive paths and/orless resistive conductive paths when it is in the LRS than when it is inthe HRS. It should be noted that resistive switching layer 104 may stillhave some conductive paths while it is in the HRS, but these conductivepaths are fewer and/or more resistive than the ones corresponding to theLRS.

When switching from its LRS to HRS, which is often referred to as areset operation, resistive switching layer 104 may release some defectsinto top electrode 102. Furthermore, there may be some mobility ofdefects within resistive switching layer 104. This may lead to thinningand, in some embodiments, breakages of conductive paths as shown in FIG.1C. Depending on mobility within resistive switching layer 104 anddiffusion through the interface formed by resistive switching layer 104and top electrode 102, the conductive paths may break closer to theinterface with bottom electrode 106, somewhere within resistiveswitching layer 104, or at the interface with top electrode 102. Thisbreakage generally does not correspond to complete dispersion of defectsforming these conductive paths and may be a self-limiting process, i.e.,the process may stop after some initial breakage occurs.

When switching from its HRS to LRS, which is often referred to as a setoperation, resistive switching layer 104 may receive some defects fromtop electrode 102. Similar to the reset operation described above, theremay be some mobility of defects within resistive switching layer 104.This may lead to thickening and, in some embodiments, reforming ofconductive paths as shown in FIG. 1B. In some embodiments, a voltageapplied to electrodes 102 and 104 during the set operation has the samepolarity as a voltage applied during the reset operation. This type ofswitching is referred to as unipolar switching. Alternatively, a voltageapplied to electrodes 102 and 104 during the set operation may havedifferent polarity as a voltage applied during the reset operation. Thistype of switching is referred to as bipolar switching. Setting andresetting operations may be repeated multiple times as will now bedescribed with reference to FIGS. 2A and 2B.

Specifically, FIG. 2A illustrates a plot of a current passing through aunipolar ReRAM cell as a function of a voltage applied to the ReRAMcell, in accordance with some embodiments. FIG. 2B illustrates the sametype of a plot for a bipolar ReRAM cell, in accordance with someembodiments. The HRS is defined by line 122, while the LRS is defined by124 in both plots. Each of these states is used to represent a differentlogic state, e.g., the HRS may represent logic one (“1”) and LRSrepresenting logic zero (“0”) or vice versa. Therefore, each ReRAM cellthat has two resistance states may be used to store one bit of data. Itshould be noted that some ReRAM cells may have three and even moreresistance states allowing multi-bit storage in the same cell.

The overall operation of the ReRAM cell may be divided into a readoperation, set operation (i.e., turning the cell “ON” by changing fromits HRS to LRS), and reset operation (i.e., turning the cell “OFF” bychanging from its LRS to HRS). During the read operation, the state ofthe ReRAM cell or, more specifically, the resistive state of itsresistance of resistance switching layer can be sensed by applying asensing voltage to its electrodes. The sensing voltage is sometimesreferred to as a “READ” voltage or simply a reading voltage andindicated as V_(READ) in FIG. 2. If the ReRAM cell is in its HRS(represented by line 122 in FIGS. 2A and 2B), the external read andwrite circuitry connected to the electrodes will sense the resulting“OFF” current (I_(OFF)) that flows through the ReRAM cell. As statedabove, this read operation may be performed multiple times withoutchanging the resistive state (i.e., switching the cell between its HRSand LRS). In the above example, the ReRAM cell should continue to outputthe “OFF” current (I_(OFF)) when the read voltage (V_(READ)) is appliedto the electrodes for the second time, third time, and so on.

Continuing with the above example, when it is desired to turn “ON” thecell that is currently in the HRS switch, a set operation is performed.This operation may use the same read and write circuitry to apply a setvoltage (V_(SET)) to the electrodes. Applying the set voltage forms oneor more conductive paths in the resistance switching layer as describedabove with reference to FIGS. 1B and 1C. The switching from the HRS toLRS is indicated by dashed line 126 in FIGS. 2A and 2B. The resistancecharacteristics of the ReRAM cell in its LRS are represented by line124. When the read voltage (V_(READ)) is applied to the electrodes ofthe cell in this state, the external read and write circuitry will sensethe resulting “ON” current (I_(ON)) that flows through the ReRAM cell.Again, this read operation may be performed multiple times withoutswitching the state of the ReRAM cell.

At some point, it may be desirable to turn “OFF” the ReRAM cell bychanging its state from the LRS to HRS. This operation is referred to asa reset operation and should be distinguished from set operation duringwhich the ReRAM cell is switched from its HRS to LRS. During the resetoperation, a reset voltage (V_(RESET)) is applied to the ReRAM cell tobreak the previously formed conductive paths in the resistance switchinglayer. Switching from a LRS to HRS is indicated by dashed line 128.Detecting the state of the ReRAM cell while it is in its HRS isdescribed above.

Overall, the ReRAM cell may be switched back and forth between its LRSand HRS many times. Read operations may be performed in each of thesestates (between the switching operations) one or more times or notperformed at all. It should be noted that application of set and resetvoltages to change resistance states of the ReRAM cell involves complexmechanisms that are believed to involve localized resistive heating aswell as mobility of defects impacted by both temperature and appliedpotential.

In some embodiments, the set voltage (V_(SET)) is between about 100 mVand 10V or, more specifically, between about 500 mV and 5V. The lengthof set voltage pulses (t_(SET)) may be less than about 100 millisecondsor, more specifically, less than about 5 milliseconds and even less thanabout 100 nanoseconds. The read voltage (V_(READ)) may be between about0.1 and 0.5 of the write voltage (V_(SET)). In some embodiments, theread currents (I_(ON) and I_(OFF)) are greater than about 1 mA or, morespecifically, is greater than about 5 mA to allow for a fast detectionof the state by reasonably small sense amplifiers. The length of readvoltage pulse (t_(READ)) may be comparable to the length of thecorresponding set voltage pulse (t_(SET)) or may be shorter than thewrite voltage pulse (t_(RESET)). ReRAM cells should be able to cyclebetween LRS and HRS between at least about 10³ times or, morespecifically, at least about 10⁷ times without failure. A data retentiontime (t_(RET)) should be at least about 5 years or, more specifically,at least about 10 years at a thermal stress up to 85° C. and smallelectrical stress, such as a constant application of the read voltage(V_(READ)). Other considerations may include low current leakage, suchas less than about 40 A/cm² measured at 0.5 V per 20 Å of oxidethickness in HRS.

Examples of ReRAM Cells

FIG. 3 illustrates a schematic representation of a ReRAM cell 300, inaccordance with some embodiments. ReRAM cell 300 may include firstelectrode layer 302, a defect source layer 304, a defect acceptor layer306, and a defect blocking layer 308. In this example, defect blockinglayer 308 is also operable as a second electrode. However, in someembodiments, a defect blocking layer and an electrode may be separatecomponents. The “first” and “second” terminology is used herein only fordifferentiating reasons and does not imply any deposition order orspatial orientation of the layers unless specifically noted.

Defect source layer 304 and defect blocking layer 308 may be formed frommaterials having different stability. For purposes of this disclosure,stability is defined as the ability of a layer to accept and releasevarious materials or vacancies, such as defects. Stable layers are lessprone, in comparison to unstable layers, to accept and release defectswhether or not these defects are present in the layer. It should benoted that a stable layer may be converted into an unstable layer byintroducing defects into this layer. For example, a stable aluminumoxide layer may be converted into an unstable aluminum oxynitride layerby plasma nitridation and other techniques.

In some embodiments, defect source layer 304 is less stable than defectblocking layer 308, which results in defect source layer 304 bring ableto release defects into defect acceptor layer 306, while at the sametime defect blocking layer 308 prevents defects from escaping defectacceptor layer 306 into defect blocking layer 308. The defect transferfrom defect source layer 304 and into defect acceptor layer 306 may bedriven by concentration difference that exists after initial depositionof these layers. Some concentration gradient (but lesser one) continuesto exist even after the transfer of defects. In other words, thetransfer does not result in a uniform distribution of defects withindefect source layer 304 and defect acceptor layer 306 and defect sourcelayer 304 continues to have a higher concentration of defects thandefect acceptor layer 306 even after the transfer. After the transfer,defect acceptor layer 306 is formed so called the lowest defectconcentration zone that is believed to be responsible for resistiveswitching. This zone may be also referred to as a switching zone.

Defect source layer 304 may be formed from one or more unstablematerials. These materials may be formed by introducing a foreignelement into otherwise stable materials. In other words, the formationof defect source layer 304 may involve at least two operations:depositing an initially stable layer and the introducing defects intothe stable layer to form an unstable material, which may be alsoreferred to as a compound based material. Additional details of theseoperations are described below with reference to FIG. 4. Some examplesof such materials include aluminum oxynitride (AlON), hafnium oxynitride(HfON), zirconium oxynitride (ZrON), and silicon oxynitride (SiON).Furthermore, inherently unstable materials, such as oxides havingmultiple phases, may be used to form defect source layer 304. Someexamples of such materials include titanium oxide (TiO₂, Ti₄O₃, Ti₂O₃,and Ti₃O₅) and tantalum oxide (TaO, TaO₂, and Ta₂O₅).

The thickness of defect source layer 304 may be between about 20Angstroms and 200 Angstroms or, more specifically, between about 40Angstroms and 80 Angstroms. The average concentration of defects indefect source layer 304 (prior to transferring some of these defectsinto defect acceptor layer 306) may be between about 10 atomic percentand 40 atomic percent or, more specifically, between about 15 atomicpercent and 30 atomic percent, such as about 20 atomic percent. Someexample of defects provided within defect source layer 304 includeelements from groups IVA, VA, and VIA of the periodic table, such ascarbon, nitrogen, oxygen, silicon, phosphorous, and sulfur. Thesedefects have lower diffusion coefficients than metals and are easier toblock. In some embodiments, metals, such as titanium, aluminum,zirconium, and hafnium may be used as defects provided that effectiveblocking of these defects is achieved and/or less aggressive annealingis used.

The concentration of defects and thickness of defect source layer 304may be controlled to ensure adequate amount of the defects available fortransfer. Prior to the transfer, the defects may be distributeduniformly within defect source layer 304 or non-uniformly. For example,more defects may be present within defect source layer 304 near itsinterface with defect acceptor layer 306 than elsewhere in defect sourcelayer 304. In another example, more defects may be present within defectsource layer 304 near its interface with first electrode 302 thanelsewhere in defect source layer 304. This type of defect distributionmay be achieved by introducing defects into defect source layer 304though one of its surfaces, e.g., by plasma nitridation.

Defect acceptor layer 306 may be formed from a relatively stablematerial in comparison, for example, to defect source layer 304. Thisstability ensures that defect acceptor layer 306 will not itself becomea source of defects. Furthermore, it ensures that defect acceptor layer306 will not accept excessive amounts of defects during transfer. Insome embodiments, the materials of defect acceptor layer 306 and defectsource layer 304 have one or more material in common. For example,defect acceptor layer 306 may be formed from aluminum oxide, whiledefect source layer may be formed from aluminum oxynitride. Thematerials of defect acceptor layer 306 and defect blocking layer 308 mayalso have one or more material in common. For example, defect acceptorlayer 306 may be formed from aluminum oxide, while defect blocking layer308 may be formed from aluminum nitride. This commonality of materialsmay be used to avoid transfer of these materials between layers andcreating additional uncontrollable defects that may skew the resistiveswitching performance. Furthermore, these common materials may bepresent at comparable concentrations. For example, concentrations of oneor more materials (e.g., aluminum in the above example) in twointerfacing layers (prior to transfer of defects between the layers) maydiffer by less than 10 atomic percent or, more specifically, by lessthan 5 atomic percent. This concentration gradient may be insufficientto move significant amount of the common material. As such, the transfermay be limited to defects.

To the contrary, concentration of defects at the interface formed bydefect source layer 304 and defect acceptor layer 306 may varysubstantially and, in some embodiments, may be at least about 10 atomicpercent or, more specifically, at least about 20 atomic percent and evenat least about 30 atomic percent prior to transfer of defects betweenthese layers. This concentration gradient may be sufficient to drivedefects from defect source layer 304 and into defect acceptor layer 306.In some embodiments, defect acceptor layer 306 is substantially free ofdefects prior to transfer of defects between the layers, i.e., theconcentration of defects in defect acceptor layer 306 is less than 3atomic percent or, more specifically, less than 1 atomic percent.

As noted above, defect acceptor layer 306 may be formed fromsufficiently stable materials, such as certain stoichiometric oxides andnitrides. Some specific examples include aluminum oxide (Al₂O₃), siliconoxide (SiO₂), hafnium oxide (HfO₂), zirconium oxide (ZrO₂), titaniumnitride (TiN), tantalum nitride (TaN), and aluminum nitride (AlN).Additional examples include metal carbon nitrides, such as Ta(CN). Whenaluminum oxide is used as the blocking layer, another material may beform an acceptor layer. It should be noted that these materials do nothave multiple phases that may result in exchange of materials withadjacent layers. Examples of materials that include multiple phasesinclude titanium oxide (TiO₂, Ti₄O₃, Ti₂O₃, and Ti₃O₅) and tantalumoxide (TaO, TaO₂, Ta₂O₅) and these materials are believed to be notsuitable for defect acceptor layer 306 or for defect blocking layer 308.However, as noted above, titanium oxide and tantalum oxide are goodcandidates for defect source layer 304 and may supply oxygen vacanciesfunctioning as defects into defect acceptor layer.

The thickness of defect acceptor layer 306 may be between about 20Angstroms and 200 Angstroms or, more specifically, between about 40Angstroms and 80 Angstroms. This thickness may depend on the thicknessof defect source layer 304 and initial concentration of defects indefect source layer 304 (i.e., the defect concentration prior toannealing that causes the defect transfer) in order to controlconcentration of defects within defect acceptor layer after theannealing. In some embodiments a thickness ratio of defect acceptorlayer 306 to defect source layer 304 is between 10:1 and 1:5 or, morespecifically between 2:1 and 1:1.

Defect blocking layer 308 is also formed from a stable material.However, while defect acceptor layer 306 is capable of accepting defectsfrom defect source layer 304, defect blocking layer 308 is not capableof accepting defects from defect acceptor layer 306. This may beachieved by forming defect blocking layer 308 from a material thatalready includes defects (e.g., titanium nitride when nitrogen functionsas a defect) and/or from material that does not accept defects. Someexamples of suitable materials include certain stoichiometric oxides andnitrides, such as aluminum oxide (Al₂O₃), silicon oxide (SiO₂), hafniumoxide (HfO₂), zirconium oxide (ZrO₂), titanium nitride (TiN), tantalumnitride (TaN), and aluminum nitride (AlN). Additional examples includemetal carbon nitride, such as Ta(CN). Materials for defect blockinglayer 308 may be separated into two groups. The first group providesgeneral blocking characteristics regardless of the composition of thedefect source layer. Stoichiometric oxides with high energy bandgap(e.g., greater than 6 eV) fall into this group. Specific examplesinclude SiO₂, Al₂O₃, AlN, SiN, and SiON. The second group includeselements that otherwise serve as defects and are supplied by the defectsource layer. For example, nitrogen may be used as defects inTiN/AlOx/AlON stack. Since AlON includes nitrogen, it effectivelybecomes a blocking layer.

In some embodiments, defect blocking layer 308, defect source layer 304,and defect acceptor layer 306 all have one common element. For example,aluminum nitride may be used for defect blocking layer 308, aluminumoxide may be used for defect acceptor layer 306, and aluminum oxynitridemay be used for defect source layer 304. In addition to aluminum beinglocked within the aluminum nitride layer, very mall, if any, diffusionwill be experienced by aluminum in this example. The stack of defectsource layer 304, defect acceptor layer 306, defect blocking layer 308may be generally represented by the formula [A][B]/[C]Ox/[C]O [B], inwhich [A]=Ti, Ta, or W; [B]=N or C; [C]=Hf, Zr, Ta, Ti, or Al. In someembodiments, the defect blocking layer may be also used as an electrodeor, more specifically, as a signal line if this layer has a sufficientconductivity. The conductivity may be at least about 1 mOhm-cm.

Returning to FIG. 3, first electrode layer 302 and defect acceptor layer306 (operable as a second electrode or a separate second electrode)provide electrical connections to ReRAM cell 300. In some embodiments,first electrode layer 302 and/or second electrode layer 306 are parts ofsignal lines that extend between multiple ReRAM cells, which may becells provided in the same row or column a memory array as furtherdescribed below with reference to FIGS. 6A and 6B. In some embodiments,first electrode layer 302 and/or second electrode layer 306 may beseparate components from the signal lines. For example, first electrodelayer 302 or second electrode layer 306 may be an intermediate electrodeand additional components, such as a diode may be provided between thiselectrode and a signal line.

The electrodes may be made from conductive materials, such as n-dopedpolysilicon, p-doped polysilicon, titanium nitride, ruthenium, iridium,platinum, and tantalum nitride. The electrodes may have a thickness ofless than about 1,000 Angstroms, such as less than about 500 Angstromsand even less than about 100 Angstroms. Thinner electrodes may be formedusing ALD techniques.

In some embodiments, first electrode layer 302 does not directlyinterface with defect source layer 304. A diffusion (not shown) may bedisposed between this electrode layer and defect source layer 304. Adiffusion barrier may be made from suitable oxygen blocking materials,such as TiSiN, TaSiN, and WSiN. In some embodiments, the diffusionbarrier layer is less than about 100 Angstroms thick, for example,between 25 Angstroms and 75 Angstroms thick, such as about 50 Angstromsthick.

In some embodiments, ReRAM cell 300 includes a current limiting layer.This layer may be formed from a material having a suitable work functionfor controlling the electron flow through ReRAM cell 300. In someembodiments, the current limiting layer is used to increase or decreasethe formed barrier height at its interface with the resistive switchinglayer. This feature is used to improve current flowing characteristicsand reduce the magnitude of the switching currents. It should be notedthat these changes in the barrier height will generally not affect theratio of switching currents (I_(ON)/I_(OFF)), and thus not impactsdetectability of different resistive states.

To achieve desirable electrical and/or physical properties of thecurrent limiting layer, its composition, structure, and/or depositionconditions may be specifically controlled. In some embodiments, thecurrent limiting layer is between about 50 Angstroms and 1000 Angstromsthick, such as between about 200 Angstroms and 50 Angstroms. The currentlimiting layer may be formed from a material that has a resistivity ofbetween about 5 Ohm-cm and 500 Ohm-cm, such as between about 50 Ohm-cmand 150 Ohm-cm. In other embodiments, the current limiting layer isformed such that its resistance (R_(RL)) is between about 10 kilo-Ohmand about 10 mega-Ohm, such as between about 100 kilo-Ohm and about 1mega-Ohm.

Resistivity is an intrinsic property of the material and can becontrolled by adjusting the composition of the material. Some specificexamples include adding alloying elements or doping atoms and/oradjusting the morphological structure of the materials, (e.g., shiftingfrom amorphous to crystal structure). In some embodiments, a currentlimiting layer may include titanium oxide doped with niobium, tin oxidedoped with antimony, or zinc oxide doped with aluminum. Theconcentration of a dopant material in the base material may be betweenabout 0.5 and 25 atomic percent or, more specifically, between about 1and 10 atomic percent

Other examples of materials suitable for the current limiting layerinclude titanium nitride (Ti_(x)N_(y)), tantalum nitride (Ta_(x)N_(y)),silicon nitride (SiN), hafnium nitride (Hf_(x)N_(y)) or titaniumaluminum nitride (Ti_(x)Al_(y)N_(z)) layer. Such layer may be formedusing ALD, CVD, and PVD techniques.

Processing Examples

FIG. 4 illustrates a process flowchart corresponding to method 400 offabricating a ReRAM cell, in accordance with some embodiments. Method400 may commence with providing a substrate that may include one or morecomponents of the ReRAM cell. For example, an electrode layer may beprovided on the substrate. In some embodiments, the substrate mayinclude a defect source layer, a defect blocking layer, and/or a defectacceptor layer forming a stack. If all three layers of the stack arealready present on the provided substrate, then method 400 may proceedwith redistributing defects within the stack during operation 410.Otherwise, method 400 may proceed with forming the layers of the stack.

Operation 404 involves forming a defect source layer. The defect sourcelayer may be formed over the electrode layer or some other layer. Forexample, a defect source layer may include aluminum oxynitride. Thedefect source layer may be formed over an electrode layer. The defectsource layer may be formed using ALD, PVD, CVD, or any other suitabletechnique.

Defects may be added into the defect source layer during its depositionor after. For example, an aluminum oxide layer may be first formed ontoa substrate and then subjected to nitridation or, more specifically, toplasma nitridation using ammonia.

In some embodiments, the defect source layer is formed from aluminumoxynitride. The concentration of oxygen in this layer may be betweenabout 30 atomic percent and 50 atomic percent, such as about 40 atomicpercent, while the concentration of nitrogen in this layer may bebetween about 10 atomic percent and 30 atomic percent, such as about 20atomic percent before annealing during operation 410.

In some embodiments, defects may be introduced into the defect sourcelayer during its deposition. For example, ALD may be used to form anano-laminate that includes both aluminum nitride and aluminum oxidessub-layers. The number of aluminum nitride layers may be controlled toachieve a specific concentration of nitrogen in the resulting defectsource layer.

Method 400 may proceed with forming a defect acceptor layer duringoperation 406. The defect acceptor layer is formed directly over thedefect source layer such that the two layers form an interface. Thedefects are later transferred through this interface. The defectacceptor layer may be also formed using ALD, PVD, CVD, or other suitabletechniques. For example, an aluminum oxide layer may be formed over thedefect source layer that includes aluminum oxynitride.

Method 400 may proceed with forming a defect blocking layer duringoperation 408. The defect blocking layer is formed directly over thedefect acceptor layer such that the two layers form an interface. Insome embodiments, the defects may be also transferred through thisinterface from the defect blocking layer and into the defect acceptorlayer. The defect blocking layer may be also formed using ALD, PVD, CVD,or other suitable techniques.

The defect blocking layer may include defects and concentration ofdefects may be greater than, for example, the desired concentration ofdefects in the defect acceptor layer at the end of the fabrication(i.e., after annealing during operation 410). These defects in thedefect blocking layer help to prevent migration of defects into thislayer from the defect acceptor layer. In some embodiments, the defectblocking layer may be also used as a source of defects. However,mobility of defects within the defect blocking layer and the defectsource layer may be different. For example, the defects in the defectblocking layer may be tightly held (as, e.g., in titanium nitride) whilethe defects in the defect source layer may be more mobile (as, e.g., inaluminum oxynitride).

Overall, operations 404-408 form a stack that includes a defect sourcelayer, a defect acceptor layer, and a defect blocking layer. In thisstack, the defect acceptor layer is disposed between the defect sourcelayer and the defect blocking layer. In some embodiments, the order ofthe operations may be reversed such that the defect blocking layer isformed first, then the defect acceptor layer is formed over the defectblocking layer, and finally the defect source layer is formed over thedefect blocking layer. In this example, the defects may be introducedinto the defect source layer during its deposition as opposed to apost-deposition treatment. For example, the defect source layer may beformed as a nano-laminate using ALD techniques. The distribution ofdefects in the nano-film may be controlled by using a specific order ofdifferent sub-layers forming this nano-laminate. Without beingrestricted to any particular theory, it is believed that combininglayers having different compositions (e.g., aluminum oxide and aluminumnitride) into a nano-laminate will create an unstable layer. In anotherexample, the defect source layer may be formed by sputtering a metal(e.g., aluminum, hafnium, zirconium) in an environment containing oxygenand nitrogen. In yet another example, the defect source layer may beformed from a metal oxide capable of changing phases (e.g., titaniumoxide. Tantalum oxide) by sputtering this metal in an oxygen containingenvironment.

FIG. 5A illustrate a schematic representation of a ReRAM cell 500including a defect source layer 504, a defect acceptor layer 506, and adefect blocking layer 508 before annealing, which transfers defects fromdefect source layer 504 to defect acceptor layer 506, in accordance withsome embodiments. FIG. 5C is an illustrative defect concentrationprofile within this cell as a function of the location within the cellbefore the annealing. Specifically, defect acceptor layer 506 may besubstantially free of defects at this fabrication stage. Defect sourcelayer 504 and defect blocking layer 508 both have defects.

Method 400 may proceed with annealing, during which defects aretransferred from the defect source layer to the defect acceptor layerduring operation 410. Operation 410 may involve a conventionalactivation anneal used for activating diodes, such exposing the stack to750° C. for 1 minute. In some embodiments, operation 410 may involveexposing the stack to between 500° C. to 800° C. for between 30 secondsand 60 minutes or, more specifically to between about 500° C. and 600°C. for between about 10 minutes and 40 minutes, such as about 550° C.for between about 20 and 30 minutes. Furthermore, operation 410 mayinvolve multiple annealing such as one performed at a high temperature(e.g., a conventional activation anneal) and another one performed alower temperature and longer duration that are specifically tailored totransfer defects from the defect source layer into the defect acceptorlayer in a controlled manner.

In some embodiments, only defects or, more specifically, only one typeof defects (e.g., one element) is moving into the defect acceptor layerduring operation 410. Other materials or vacancies are not transferredbetween the layers during this operation. For example, the transfer rateof other elements may be less than 10% that of the defects. As statedabove, the mobility of other elements may be restricted by havingsimilar concentrations of these elements in adjacent layers or lockingthe elements with the layer by, for example, covalent bonding.

In some embodiments, the defect blocking layer includes a metal andsubstantially no metal is transferred from the defect blocking layer tothe defect acceptor layer during transferring the defects from thedefect source layer to the defect acceptor layer.

FIG. 5B illustrate a schematic representation of a ReRAM cell 510including a defect source layer 514, a defect acceptor layer 516, and adefect blocking layer 508 after the annealing that transfers defectsfrom the defect source layer to the defect acceptor layer, in accordancewith some embodiments. The defect source layer is now identified as 514,and the defect acceptor layer is now identified as 516 because of thechanges in the layers that have occurred during the annealing, i.e., thedefect source layer losing the defects while the defect acceptor layeraccepting the defects. There may be few if any changes in the defectblocking layer. As such, this layer is still identified as 508. FIG. 5Dis an illustrative defect concentration profile within this cell as afunction of the location within the cell after the annealing.Specifically, defect acceptor layer 516 now has defects that havetransferred from defect source layer 514 and, in some embodiments, fromthe defect blocking layer. Still, the concentration of defects in defectacceptor layer 516 is less than that in defect source layer 514. Assuch, defect acceptor layer 516 has the minimum defect concentrationzone, which is believed to be responsible for switching. The defectconcentration may be higher on each side of this zone.

To contract distribution of defects within described ReRAM cells, FIG.5E illustrates a schematic representation of a ReRAM cell 520 that doesnot include a defect blocking layer, in accordance with someembodiments. As such, when defect source layer 528 supplies defects intodefect acceptor layer 526 the defects continue to flow into otherlayers, such as layer 522. FIG. 5F is an illustrative defectconcentration profile as a function of the location within a ReRAM cellfor this type of cells.

Memory Array Examples

A brief description of memory arrays will now be described withreference to FIGS. 6A and 6B to provide better understanding to variousaspects of thermally isolating structures provided adjacent to ReRAMcells and, in some examples, surrounding the ReRAM cells. ReRAM cellsdescribed above may be used in memory devices or larger integratedcircuits (IC) that may take a form of arrays. FIG. 6A illustrates amemory array 600 including nine ReRAM cells 602, in accordance with someembodiments. In general, any number of ReRAM cells may be arranged intoone array. Connections to each ReRAM cell 602 are provided by signallines 604 and 606, which may be arranged orthogonally to each other.ReRAM cells 602 are positioned at crossings of signal lines 604 and 606that typically define boundaries of each ReRAM cell in array 600.

Signal lines 604 and 606 are sometimes referred to as word lines and bitlines. These lines are used to read and write data into each ReRAM cell602 of array 600 by individually connecting ReRAM cells to read andwrite controllers. Individual ReRAM cells 602 or groups of ReRAM cells602 can be addressed by using appropriate sets of signal lines 604 and606. Each ReRAM cell 602 typically includes multiple layers, such as topand bottom electrodes, resistance switching layer, embedded resistors,embedded current steering elements, and the like, some of which arefurther described elsewhere in this document. In some embodiments, aReRAM cell includes multiple resistance switching layers provided inbetween a crossing pair of signal lines 604 and 606.

As stated above, various read and write controllers may be used tocontrol operations of ReRAM cells 602. A suitable controller isconnected to ReRAM cells 602 by signal lines 604 and 606 and may be apart of the same memory device and circuitry. In some embodiments, aread and write controller is a separate memory device capable ofcontrolling multiple memory devices each one containing an array ofReRAM cells. Any suitable read and write controller and array layoutscheme may be used to construct a memory device from multiple ReRAMcells. In some embodiments, other electrical components may beassociated with the overall array 600 or each ReRAM cell 602. Forexample, to avoid the parasitic-path-problem, i.e., signal bypasses byReRAM cells in their low resistance state (LRS), serial elements with aparticular non-linearity must be added at each node or, morespecifically, into each element. Depending on the switching scheme ofthe ReRAM cell, these elements can be diodes or varistor-type elementswith a specific degree of non-linearity. In the same other embodiments,an array is organized as an active matrix, in which a transistor ispositioned at each node or, more specifically, embedded into each cellto decouple the cell if it is not addressed. This approach significantlyreduces crosstalk in the matrix of the memory device.

In some embodiments, a memory device may include multiple array layersas, for example, illustrated in FIG. 6B. In this example, five sets ofsignal lines 614 a-b and 616 a-c are shared by four ReRAM arrays 612a-c. As with the previous example, each ReRAM array is supported by twosets of signal lines, e.g., array 612 a is supported by 614 a and 616 a.However, middle signal lines 614 a-b and 616 b, each is shared by twosets ReRAM arrays. For example, signal line set 614 a providesconnections to arrays 612 a and 612 b. Top and bottom sets of signallines 616 a and 616 c are only used for making electrical connections toone array. This 3-D arrangement of the memory device should bedistinguished from various 3-D arrangements in each individual ReRAMcell.

Experimental Data

An experiment was conducted to determined performance of ReRAM cells,each built with a defect source layer, a defect acceptor layer, and adefect blocking layer. This performance was compared with conventionalcells that did not include at least one of these layers, i.e., controlcells. Two types of control cells were used in this experiment. One typeof cell included a resistive switching layer formed from aluminum oxide(Al₂O₃) without any defects added during or after its deposition.Another type of control cell included a resistive switching layer formedfrom aluminum oxynitride (AlON) by converting an initial depositedaluminum oxide layer into the aluminum oxynitride. Ammonia-based plasmanitridation was used for this conversion.

The test cells included a defect source layer containing aluminumoxynitride and a defect acceptor layer containing aluminum oxide. Theselayers were deposited in a manner similar to the first control cells(for the defect acceptor layer) and in a manner similar to the secondcontrol cells (for the defect source layer). In other words, each testcell included a resistive switching layer of a first control celloperable as a defect acceptor layer and a resistive switching layer of asecond control cell operable as a defect source layer.

All cells (both types of control cells and test cells) included n-dopedpolysilicon electrodes on one side and titanium nitride electrodes onthe other side. In the test cells, the n-doped polysilicon electrodesinterfaced the defect source layers, while the titanium nitride layersinterfaced the defect acceptor layer. All cells were subject to annealat 750° C. for 1 minute prior to testing their resistive switchingcharacteristics.

Resistive switching characteristics of these three types of cells werethen tested. The cells were switched between their high resistive statesand low resistive states using bipolar switching methodology and thehighest switching voltages have been identified. The following testingparameters were used: Start Voltage=1 V, voltage increase steps=0.2V,Pulse during=100 ns. The test cells demonstrated a lower reset voltage(i.e., a voltage used to switch from the low resistive state to the highresistive state) than both types of control cells. Specifically, theaverage reset voltage for the test cell was 5.8V, while the averagereset voltage for the first type of the control cells (i.e., the cellswith aluminum oxide resistive switching layers) was 6.3V and the averagereset voltage for the second type of the control cells (i.e., the cellswith aluminum oxynitride resistive switching layers) was 7.6V. In otherwords, the reduction in the reset voltage was 8% and 25% respectivelyfor the test cells in comparison to the two types of the control cells.It is believed that further improvement could be achieved byspecifically tailoring relative thicknesses of the defect source layerand the defect acceptor layer.

Distribution of nitrogen and oxygen within the test cells was studiedusing secondary ion mass spectrometry (SIMS). FIG. 7 illustrates plotsof a nitrogen concentration 702 and an oxygen concentration 704 as afunction of the location within a test cell after its annealing. Thelocation of the defect blocking layer (i.e., the titanium nitrideelectrode) is identified as 710, the location of the defect acceptorlayer is identified as 712, the location of the defect source layer isidentified as 714, and finally the location of the doped polysiliconelectrode is identified as 716. Between the defect acceptor layer 712and the defect source layer 714, the lowest nitrogen concentration waswithin the defect acceptor layer 712 and identified with block 706. Thislocation within the cell connection to block 706 may be referred to asthe lowest defect concentration zone. Without being restricted to anyparticular theory, it is believe that the resistive switching occurswithin this zone. The fact that nitrogen concentration 702 tapers downwithin doped polysilicon electrode is irrelevant since nitrogen is notcapable of sufficiently impact resistance of this layer

CONCLUSION

Although the foregoing examples have been described in some detail forpurposes of clarity of understanding, the invention is not limited tothe details provided. There are many alternative ways of implementingthe invention. The disclosed examples are illustrative and notrestrictive.

What is claimed:
 1. A method of fabricating a resistive random accessmemory (ReRAM) cell, the method comprising: providing a stack comprisinga defect source layer, a defect blocking layer, and a defect acceptorlayer, the defect acceptor layer being disposed between the defectsource layer and the defect blocking layer; and annealing the stack,wherein the annealing transfers defects from the defect source layer tothe defect acceptor layer and creates a resistive switching zone withinthe defect acceptor layer, and wherein substantially no defects aretransferred from the defect acceptor layer to the defect blocking layer.2. The method of claim 1, wherein the annealing is performed at atemperature of between 500° C. and 800° C. for between 30 seconds and 30minutes.
 3. The method of claim 1, wherein the defects comprise one ofoxygen, nitrogen, carbon, or silicon.
 4. The method of claim 1, whereinthe defect blocking layer comprises the defects.
 5. The method of claim4, wherein some of the defects in the defect blocking layer aretransferred to the defect source layer.
 6. The method of claim 1,wherein the defect blocking layer comprises a metal, and whereinsubstantially no metal is transferred from the defect blocking layer tothe defect source layer.
 7. The method of claim 1, wherein the defectblocking layer comprises one of silicon oxide, aluminum oxide, hafniumoxide, zirconium oxide, titanium nitride, tantalum nitride, or aluminumnitride.
 8. The method of claim 1, wherein the defect blocking layer isoperable as an electrode.
 9. The method of claim 1, wherein the defectacceptor layer and the defect source layer comprise a same metal. 10.The method of claim 1, wherein the defect source layer comprises one ofAlO_(X)N_(Y), HfO_(X)N_(Y), ZrO_(X)N_(Y), SiO_(X)N_(Y), TiO_(X), orTaO_(X).
 11. The method of claim 1, wherein the defect source layercomprises AlO_(X)N_(Y) and the defect acceptor layer comprises Al₂O₃prior to transferring the defects from the defect source layer to thedefect acceptor layer.
 12. The method of claim 11, wherein the defectsource layer comprises between about 10 atomic percent and 30 atomicpercent of nitrogen prior to transferring the defects from the defectsource layer to the defect acceptor layer.
 13. The method of claim 12,wherein nitrogen is distributed non-uniformly throughout a thickness ofthe defect source layer prior to transferring the defects from thedefect source layer to the defect acceptor layer.
 14. The method ofclaim 13, wherein more nitrogen is present within the defect sourcelayer near the interface between the defect source layer and the defectacceptor layer than elsewhere in the defect source layer prior totransferring the defects from the defect source layer to the defectacceptor layer.
 15. The method of claim 11, wherein the resistiveswitching zone comprises between about 1 atomic percent and 15 atomicpercent of nitrogen on average after transferring the defects from thedefect source layer to the defect acceptor layer.
 16. The method ofclaim 15, wherein the resistive switching zone comprises less than 10atomic percent of nitrogen on average after transferring the defectsfrom the defect source layer to the defect acceptor layer.
 17. Themethod of claim 11, wherein the resistive switching zone has a lowestnitrogen concentration within the defect acceptor layer and the defectsource layer.
 18. The method of claim 17, wherein the resistiveswitching zone comprises the lowest nitrogen concentration within thedefect acceptor layer and the defect blocking layer.
 19. A method offabricating a resistive random access memory cell, the methodcomprising: providing a first layer comprising aluminum oxide, the firstlayer having a thickness of between 20 Angstroms and 100 Angstroms;introducing nitrogen into the first layer, wherein introducing nitrogencomprises plasma nitridation using ammonia and wherein the first layercomprises between about 15 atomic percent and 25 atomic percent onaverage after introducing nitrogen. depositing a second layer comprisingaluminum oxide over the first layer, the second layer having a thicknessof between 20 Angstroms to 100 Angstroms; depositing a third layercomprising titanium nitride over the second layer; and annealing thefirst layer, the second layer, and the third layer at a temperaturebetween 700° C. and 800° C. for a time period between 0.5 minutes and 2minutes, wherein annealing results in transferring nitrogen from thefirst layer and into the second layer while substantially no nitrogen istransferred from the second layer and into the third layer, and wherein,after annealing, the second layer has a lowest nitrogen concentrationzone between the first layer and the second layer, the lowest nitrogenconcentration zone having a nitrogen concentration of less than 10atomic percent.
 20. A resistive random access memory comprising: a firstlayer comprising aluminum, oxygen, and nitrogen, the first layer havinga thickness of between about 20 Angstroms to about 100 Angstroms; asecond layer comprising aluminum, oxygen, and nitrogen, the second layerdisposed over the first layer, the second layer having a thickness ofbetween about 20 Angstroms to about 100 Angstroms; and a third layercomprising titanium nitride, the third layer disposed over the secondlayer, wherein the second layer has a lowest nitrogen concentration zonewithin the first layer and the second layer, the lowest nitrogenconcentration zone having a nitrogen concentration of less than 10atomic percent.